1. Field of the Invention
This invention relates to an electrophotographic photosensitive member, and more particularly to an electrophotographic photosensitive member of selenium type comprising selenium containing oxygen.
2. Description of the Prior Art
Construction of an electrophotographic photosensitive member varies depending upon the desired characteristics or the type of the desired electrophotographic process, but in all cases the electric charge of electron-hole pairs caused by injection of electromagnetic waves is drifted under an electric field, and the performance of electrophotographic photosensitive member largely depends on the photoconductive layer.
In case of photosensitive members having an insulating layer on the surface of the photoconductive layer, electrostatic images are formed on the surface of the insulating layer, and therefore, it is necessary that electric charges are injected to the interface between the insulating layer and the photoconductive layer.
An example of electrophotographic processes applicable to such photosensitive members as above comprises a primary charging, an imagewise exposure simultaneously with AC discharging or charging in a polarity opposite to the polarity of the primary charging, or the AC discharging or the charging being effected after an imagewise exposure, and a blanket exposure.
When the photoconductive layer is composed of a p-type semiconductor, such as Se, SeTe and the like, the primary charging is carried out by a negative corona discharging to inject positive charges into the photoconductive layer from the support resulting in transferring the charges to the interface between the insulating layer and the photoconductive layer by the electric field applied to the photoconductive layer. When it is difficult to inject electric charges from a support, the photosensitive member is uniformly irradiated from the support side with a light immediately before or simultaneously with a negative corona discharging so as to form a proper amount of positive charge at the interface between the insulating layer and the photoconductive layer. When the light irradiation is effected from the support side, the support should be composed of a light transmissive material such as Nesa glass, transparent resin films and the like.
In case of photosensitive members which do not have an insulating layer on the photoconductive layer, it is not necessary to inject electric charges upon the primary charging. A typical process therefor where a p-type semiconductor is employed comprises applying an electric field to the photoconductive layer by a primary charging with a positive corona discharging and transferring positive charges formed by imagewise exposure to the support side.
Heretofore, Se-type photoconductive layers have been known to be of high sensitivity and high mechanical strength which may be produced by vapor-deposition of Se or Se containing As, Te, Sb or the like, but have some drawbacks to be solved. One of the drawbacks is a fatigue due to repeating.
When a photosensitive member is used repeatedly, the difference in potential between a dark portion and a light portion of an electrostatic image becomes smaller than that at the beginning, and therefore, it is observed that the contrast is lowered. This is due to so-called fatigue of photosensitive members. It seems that electric charges are trapped in a photoconductive member due to electric defects in the photoconductive layer and become space charges. This phenomenon is increased as the photosensitive member is used repeatedly. As a result, the residual potential increases and the potential at dark portions decreases.
For the purpose of producing electrostatic images of high contrast with good reproducibility, a photoconductive layer free from the fatigue due to repeating is indispensable.